Although the pharmacological profiles and clinical efficacy of antihistamines for patients with atopic dermatitis (AD), one of the representative cutaneous pruritic diseases, have been well documented, the in vivo concentrations of antihistamines in human skin have previously been studied in less detail. In this randomized trial, the suction blister technique was applied to the measurement of the concentrations of epinastine hydrochloride in the extracellular water compartment in comparison with chlorpheniramine maleate in skin from AD patients. A total of 79 patients (mean age, 28.6 years) were randomly allocated to receive either 20 mg of epinastine or 6 mg of chlorpheniramine. Suction blisters were induced on both upper arms in all patients, and blister fluid was obtained for the measurement of concentrations of the 2 test agents by liquid chromatography-tandem mass spectrometry. Epinastine concentrations in 42 samples were 5.02-33.07 ng/mL (mean ± SD, 14.08 ± 10.51; median, 7.00), demonstrating that epinastine is distributed to the skin in high concentrations equal to the levels found in plasma and sufficient to exert its variety of pharmacological modes of action. In contrast, chlorpheniramine concentrations in all 37 samples were below the lower limit of quantification (< 0.5 ng/mL). Corresponding to these pharmacological results, a significant decrease of pruritus was observed in AD patients administered epinastine compared with chlorpheniramine. Hence epinastine is likely to be more effective clinically than chlorpheniramine in AD. This is the first report for the determination of in vivo local drug levels of antihistamines in the skin from AD patients.